In measurement systems using harmonic down conversion to reduce a radio frequency (RF) source signal to a lower, and hence more tractable, intermediate frequency (IF) signal for measurement purposes, a major source of measurement error is harmonic skip. Harmonic skip occurs when different harmonics of the local oscillator signal which is used to convert the radio frequency signal to the intermediate frequency signal are used each time in arriving at the intermediate frequency signal. And when harmonic skip occurs, the measured intermediate frequency signal fluctuates in amplitude and phase in an unpredictable manner.
In the prior art, several approaches are used to counter this harmonic skip problem. One approach is to require the operator to "tune" the source signal. Another approach is to require the operator to tune the local oscillator. Both approaches, as is evident, necessitates input from the operator to stabilize the signals for the least amount of harmonic skipping and are workable only for continuous wave (CW) signals. When a swept signal is used, as in must systems, these techniques become approximations.
In accordance with the present invention, harmonic skipping is eliminated in a down-conversion system without requiring input from the operator. In accordance with the preferred embodiment of the invention, a pretune controller varies and sets a local oscillator signal to the proper frequency for mixing with and down converting the radio frequency source signal to a predetermined intermediate frequency signal. In the down conversion, a predetermined harmonic number of the local oscillator signal is used. A predetermined harmonic of the source signal may also be used; in such cases, the source signal frequency is varied by a controller.
In the case of a predetermined harmonic of the local oscillator, the pretune control setting is verified as the proper one by requiring the local oscillator to change in frequency by a predetermined step. The corresponding change in the intermediate frequency signal should be the same as the product of the predetermined harmonic number and the step change. In other words, EQU IF Change=(Harmonic Number).times.(Step Change).
If the harmonic number inadvertently differs from the predetermined one, the corresponding intermediate frequency signal change will then differ from the product. In such an event, the pretune control is made to vary, for example, by a microprocessor, until the intermediate frequency signal is again produced, thus enacting a "searching" process. The local oscillator is again made to change in frequency by a predetermined step, the corresponding intermediate frequency signal change is measured, and a comparison of this change to the product is again made. This process is repeated until the corresponding change is equal to the product of the step change and the harmonic number. When this happens, the proper local oscillator frequency is achieved and the predetermined harmonic of the local oscillation is used in the down conversion. No harmonic skipping is occurring to invalidate measurements.